1. Field of the Invention
The present invention relates to a method and composition for stimulating skeletal growth in normal poultry.
More particularly the present invention relates to a method and composition for stimulating growth in normal poultry involving the administration of a pituitary peptide hormone.
2. Description of Related Art
Physiological action on tissues by one of the pituitary peptide hormones is usually essential for growth in vertebrates. One such peptide, Growth Hormone (GH, also known as somatotrophin), has attracted most attention, partly because of its proved importance in human growth. It also stimulated growth in studies on other (mainly mammalian) species. Prolactin (PRL) is a similar pituitary peptide. Due to its stimulus to the development of mammary tissue, PRL has usually been regarded as a lactogenic hormone. In birds, it has been studied particularly for its part in reproductive and maternal behavioural processes. Thus it is involved in fattening of migratory species, influences brooding and stimulates production of pigeon 'milk'(sloughed gastric mucosa). Indeed, this forms the basis of a classical test for lactogenicity, used to this day. Both GH and PRL have functions additional to those mentioned above. These vary from class to class and species to species and may be contradictory when comparing species with species. The relative roles of the two may also be exchanged between them. It is not surprising then, that there is a considerable degree of homology of amino-acid sequence between GH PRL. This is only 50% in humans, but higher in some other species. Nevertheless, with respect to the possible effects of each of the two hormones, interest in and positive evidence about the role of GH in growth is paramount, particularly in mammals. However, PRL stimulates tissue preservation and renewal, and growth and development in amphibians and reptiles, the latter class being phylogenetically close to birds. It also stimulates growth in juveniles of some rodent species.
Increase of weight alone (perhaps due to fat) without change in size, is inadequate evidence for growth stimulation by hormones. This is an important consideration when studying juvenile growth. The latter has been studied particularly in humans, GH having a positive growth effect and PRL, none. Hormonal influence on bird growth had been studied relatively little, until interest increased in recent years, because of the commercial potential of improved poultry growth. Work has been concentrated on GH. Initially, because of the difficulty in obtaining native hormone, studies were performed with bovine or ovine GH. Stimulation of growth was either absent or uncertain, although body fat seemed to be clearly reduced. Chicken GH is now more available. Produced in bacteria by several groups, using recombinant genetic technology, chicken GH has been administered with disappointing results. The effect on the growth of whole poultry birds by PRL administered alone, has not been known.
In an article published by D.B. King and C.G. Scaner in June 1986 (Proc. Soc. Expt. Biol. Med. 182:201-207) entitled Effect of Mammalian Growth Hormone and Prolactin on the Growth of Hypophysectomized Chickens, it is stated at the end of the abstract that "These results provide evidence that mammalian GH enhances body weight gain, bone growth and the growth of several organs in the hypophysectomized chicken. Mammalian PRL increased body weight gain, liver weight and adipose tissue weight in corticosterone-treated hypophysectomized chickens, but did not influence bone growth or the weights of the heart, pectoralis, thymi or bursa".
It is first to be noted that while said article teaches that administration of growth hormone can partially restore growth in an animal with restricted growth, there is no certainty that the same treatment will improve on normal growth in a normal animal. Nevertheless, one might, based on these results, try to stimulate poultry growth of normal poultry using growth hormone.
On the other hand, in light of the clear statement in said article that "mammalian PRL ... did not influence bone growth", one would naturally assume that PRL also would not influence bone growth in normal poultry.
It was, thus, extremely surprising to discover that contrary to the negative teaching of said article, prolactin in fact was effective for stimulating skeletal growth in normal poultry, as evidenced by increased metatarsal length as described hereinafter.